The present invention relates to data storage devices. More specifically, the present invention relates to a write-once thin-film data storage device.
New types of non-volatile, thin-film memory include Magnetic Random Access Memory ("MRAM") based on spin dependent tunneling ("SDT") junctions. A typical SDT junction has a pinned ferromagnetic layer, a sense ferromagnetic layer and a thin insulating tunnel barrier sandwiched between the ferromagnetic layers. A logic value may be written to an SDT junction by applying a magnetic field that sets the SDT junction's magnetization orientation to parallel (logic value `0`) or anti-parallel (logic value `1`). Relative orientation and magnitude of spin polarization of the ferromagnetic layers determine the resistance state (R or R+.DELTA.R) of the SDT junction. The logic value stored in the SDT junction may be read by sensing the resistance of the SDT junction.
Another type of non-volatile, thin-film memory is polymer memory based on polar conductive polymer molecules. Data is stored as a "permanent polarization" in a polymer molecule (in contrast to an SDT junction, where data is stored as a `permanent magnetic moment`). Polymer memory elements may be written by applying electric fields. The resistance state (R or R+.DELTA.R) of a polymer memory element is dependent upon the orientation of polarization of the polymer molecules. Polymer memory elements may also be read by sensing their resistance.
These thin-film memory elements are re-writable. That is, data can be written to these elements many times.
Write-once memory, in contrast, allows data to be written only once. Write-once memory is commonly used to permanently store key chip information such as chip/manufacture ID, access codes and error maps.
Typical devices for storing key chip information include erasable programmable read-only memory ("EPROM") devices and fuse programmable devices. However, EPROM and fuse programmable devices do not offer the high density and low voltage features of MRAM and polymer memory devices. Moreover, EPROM and fuse programmable devices are not easily integrated with MRAM and polymer memory devices.